The typical refrigerator or heat pump includes an evaporator, a compressor, a condenser, and an expansion valve or capillary tube. As is well known, the working fluid which is referred to as the refrigerant goes through a thermodynamic cycle. The refrigerant leaves the compressor as a vapor at an elevated pressure and then condenses resulting in the transfer of heat to cooling water or fins surrounding the condenser. The high pressure liquid passes through an expansion valve or capillary tube where some of the liquid flashes into vapor. The remaining fluid is vaporized in the low pressure evaporator resulting in the transfer of heat from the refrigerated volume. This vapor then enters the compresser and the cycle begins again. In short, in the evaporator, the refrigerant absorbs heat from the surroundings and, in the condenser, it gives heat off. During the cooling cycle, ice builds up on the coils and fins of the evaporator because the temperature of the evaporator is substantially below the freezing point of water. As is well known, this ice substantially reduces the coefficient of performance of the system. More specifically, the ice acts as an insulator and provides a thermal barrier that interferes with the thermal transfer to the evaporator. Accordingly, the compresser has to work harder and/or longer to provide the required thermodynamic cycle. Also, energy is lost to heat of solidification in forming the ice.
There are two common prior art approaches to removing ice from the evaporator automatically. Both approaches require the expenditure of substantial amounts of energy. The first approach which is used extensively for automatic defrosting refrigerators involves the use of heat from an external source to melt the ice. Typically, a resistive heating element is connected to the evaporator or mounted in a position adjacent thereto. Then, in response to a timer, electric current is passed through the element during an off cycle of the compresser. The period for ice to build up on the evaporator is a function of several parameters such as, for example, the season of the year. Typically, however, most automatic defrost refrigerators activate the defrost cycle at a constant interval in the range from 10 to 16 hours. Because a typical defrost heating element may be rated at 1.2 Kw per hour and the average cycle time may be 15 minutes or longer, it is not uncommon for a refrigerator to use up to 1 Kw each day in defrosting the evaporator. This excessive amount of energy is inefficiently used to melt the ice because a substantial percentage of the heat may be radiated into the freezer. Further, this defrosting energy does not include the energy required to cool the evaporator and freezer back down to the steady state operating condition. It follows that the use of an external heat source to defrost the evaporator may significantly reduce the coefficient of performance.
The second common prior art approach to defrosting an evaporator is used extensively in relatively large capacity heat pumps. A four-way valve is employed in conjunction with the compressor so that during the defrost cycle, the direction of flow of the refrigerant can be reversed. In essence, the evaporator becomes the condenser and the condenser becomes the evaporator. As described earlier herein, heat is given off in the condenser stage of the cycle and this heat is used to melt the ice. One draw back of this defrosting approach is the initial cost of the four-way valve and controls. Also, although this approach utilizes less energy than operating a heating element, some energy is expended driving the compressor during the reverse operation of the defrost cycle.